Unite for Europe, an umbrella group of remain campaigners, plans to march on parliament at 11am on 25 March – the last weekend before Theresa May’s self-imposed deadline for launching the process of leaving the European Union – hoping to attract a crowd in excess of three-quarters of a million people.

“I don’t think we were ever under the illusion that the march would stop article 50 being triggered; it is more about demonstrating the strength of opinion against Brexit,” said Peter French of Unite for Europe. “This fast-track bill will create a lot of emotion and anger in the country that things are being rushed through.”

A march against the Iraq war in 2003 is thought to be the largest in modern times, attracting 750,000 people according to police estimates and up to 2 million according to organisers at the time. In contrast, the largest anti-Brexit march to date attracted an estimated crowd of 50,000 on 2 July, shortly after the EU referendum, and there have been two smaller marches in August and September.

“The idea is to try to make this the biggest march the capital, or country, has ever seen,” said French, who pointed out that there were 6 or 7 million voters in London alone, some 70% of whom had opposed Brexit.

Thousands of demonstrators march through Whitehall to protest against the war in Iraq in 2003. Photograph: Sipa Press / Rex Features

“Right now, Theresa May is focused entirely on the 52% and forgetting the 48%,” he said. “We aim to impress on government the strength of feeling about the government rushing this through with no sense of what it will do to the country.”

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But the “Stop Brexit” protest organisers are undeterred by the possibility that article 50 may have already been invoked by 25 March and say it underlines their concern that the government is ignoring growing concern about the consequences of leaving the EU.

The date of the planned demonstration is also the 60th anniversary of the Treaty of Rome, which founded the European Economic Community, and organisers hope it will attract a broad spectrum of EU citizens and young people who were unable to vote in the referendum.

“This is a march for everyone: people who voted against, who were wavering, or perhaps those who wanted to leave but are now finding out that abandoning the single market and customs union is not what they voted for,” said French.

Resistance is also growing in Westminster, where a number of Labour MPs are preparing to defy a three-line whip from party leaders and vote against the government bill, while Conservative peers are among an active group in the Lords considering what role it should play in what is expected to be a tightly fought amendment process.

But politicians advising the march have stressed the importance of also showing grassroots opposition to an overly harsh or hasty Brexit. Organisers have been particularly buoyed by the success of last weekend’s women’s marches against Donald Trump, which were seen as symbolising a new spirit of peaceful protest and defiance against the forces of political nationalism.

Women march in London to protest against Donald Trump on the day after he became US president. Photograph: G V Herrero/Barcroft Images

“We were very encouraged by the Trump march and also by the atmosphere and enthusiasm from a cross-section of society,” added French, a professional singer who has no previous involvement in politics before the referendum. “It shows that there is a still a need out there for people to gather and that it is important to get out there and be heard.”

Organisers are working with the Metropolitan police, Greater London Authority and Westminster council on a route for the march that will likely start in Park Lane before heading to Piccadilly, Trafalgar Square and end in Parliament Square with a series of as-yet unannounced speakers.

Organisers of the Stop Brexit march defended their decision to hold a single event in London, citing the logistical challenge since launching the planned march on 30 December. So far largely organised on Facebook, they are currently crowd funding and seeking volunteer marshals.

They say that they want to maximise the visual impact by staging the largest march possible. “It is vital that it is in London because we are marching on parliament; our message is to parliament,” said French.